AMMAN (Reuters) - Military jets believed to be Russian killed at least 60 civilians trying to flee heavy fighting in the oil rich Deir al Zor province of Syria when their small boats were targeted as they sought to cross the Euphrates River, opposition activists, former residents and a war monitor said late on Wednesday.

They said the jets targeted makeshift rubber dinghies and boats carrying dozens of families fleeing the town of al Ashara along the western banks of the Euphrates that lies south of Deir al Zor city, the provincial capital.

Islamic State’s last major stronghold, the cities, towns and farms in the fertile strip along the Euphrates bordering Iraq are fast becoming the focus of Syria’s six-year-long civil war.

“Russian jets staged a second wave of strikes on the boats that were fleeing across the river causing more casualties among those who rushed to rescue earlier survivors,” said Abdullah al Akaidat, a tribal figure in northern Syria from Al Ashara who is in contact with relatives in the area.

Russia is throwing military weight behind the Syrian army campaign’s to regain the province bordering Iraq, racing with U.S- backed forces to grab territory from Islamic State.

U.S. coalition jets last year destroyed bridges that linked villages on the northeast of the Euphrates river with towns on the opposite bank.

Although the goal was to cut militant supply lines, it forced people to use ferries to cross the river and its tributaries and ruptured a major lifeline for civilians, raising prices of goods and food.

Fifteen civilians were killed in air strikes during the last 24 hours on the town of al Quriya, just further north of the town of Ashara along the river, said former residents in touch with relatives.

Thousands of residents of the eastern province are fleeing war-torn zones to the safety of towns that have escaped relatively unscathed from the relentless fighting.

The Russians built a bridge across the Euphrates near Deir Zor city to move troops and equipment.

The militants waged a surprise counter offensive and claimed to have killed dozens of Syrian troops, Russian ground troops and Iranian-backed fighters. They regained some territory back.

Islamic State released a video on Tuesday of two injured men it claimed were Russian soldiers captured in Deir al Zor province. Reuters could not verify the authenticity of the tape.

The Syrian army is seeking to advance towards Mayadeen, a city in the province 44 km southeast of Deir Zor, where the river flows through it.

Mayadeen is a major Islamic State stronghold that has also been targeted by the U.S.-led coalition which had earlier this year also increased bombing of the group in cities and towns along the Euphrates valley.

Relentless airstrikes by Russian jets in Deir al Zor province have intensified in recent days according to the Syrian Observatory for Human Rights. The war monitor documented the death of at least 185 civilians, including at least 45 woman and children, in five days of aerial strikes.

Russia rejects opposition and human rights groups accusations that the bombing campaign has killed thousands of civilians since its major intervention two years ago that turned the tide in favour of Syrian President Bashar al-Assad.

Scores of hospitals and civil defence centres have been bombed in what the opposition said is a “scorched earth policy” to paralyse life in rebel-held areas. Moscow says it only attacks hardline Islamists.